1. Field of the Invention
The present invention relates to a multiphase clock generation circuit, and more particularly, to a multiphase clock generation circuit capable of changing a frequency of a multiphase clock signal to be generated.
2. Description of the Related Art
In recent years, a clock generation circuit is used in various semiconductor devices so as to generate clock signals. Examples of the clock generation circuit include a multiphase clock generation circuit capable of generating a plurality of clock signals with different phases (hereinafter, referred to as “multiphase clock signals”). The multiphase clock generation circuit is used as, for example, a clock generation circuit for a pulse width modulation circuit. The pulse width modulation circuit is a circuit for controlling a pulse width of a pulse width modulation (PWM) pulse which is generated in response to the multiphase clock signals.
The pulse width modulation circuit is used for, for example, a laser beam printer (LBP). The laser beam printer is widely used because of its high-resolution, silent, and high-speed features. In such apparatus, tone (i.e., density) is controlled based on an H width of a pulse for each dot to be printed.
Here, FIG. 12 shows a simple block diagram of the laser beam printer, and description is given of the laser beam printer. The laser beam printer controls a laser beam output from a laser beam 103 by using PWM pulse signals output from a pulse width modulation circuit 104. Then, the laser beam printer applies the laser beam onto a printing material (for example, paper) 101 through a lens 102 to thereby perform printing. Note that the printing is performed for each line by scanning the printing material with the laser beam in one direction.
FIG. 13 shows an example of printing results. As shown in FIG. 13, the printing is performed for each dot d and the tone of each dot is adjusted based on a width of a printing area p provided in each dot. In the laser beam printer, the position and width of the printing area p are controlled with the H width of the PWM pulse signal. Accordingly, in order to obtain an image with high definition, it is necessary for the pulse width modulation circuit 104 to control the pulse width with high precision. In order to achieve such performance, the pulse width modulation circuit 104 controls, with high precision, the pulse width of the PWM pulse signal to be output using the multiphase clock signals.
JP 2006-20109 A and US 2006/0001467, which are assigned to the same assignee as the present invention, each disclose an example of the pulse width modulation circuit 104. FIG. 14 shows a block diagram of the pulse width modulation circuit 104. The pulse width modulation circuit 104 generates multiphase clock signals with 256 phases based on a reference clock signal. Then, a synchronous position detection circuit 120 detects which multiphase clock signal is synchronized with a horizontal synchronous signal to be used as a reference for starting a scan, and outputs a synchronous position detection signal. A digital pulse data signal processing circuit 130 converts input digital pulse data into rise information or fall information of the PWM pulse based on synchronous position detection results so as to synchronize the horizontal synchronous signal with the PWM pulse. A multiphase clock selection circuit 140 selects a specified clock signal from among the signals contained in the multiphase clock signals based on the rise information and the fall information. A pulse width modulation signal generation circuit 150 generates a pulse width modulation signal (PWM pulse) based on the selected clock signal.
Specifically, the pulse width modulation circuit 104 enables control of the pulse width of the PWM pulse signal with high precision by using the multiphase clock signals. However, in the laser beam printer incorporating a plurality of drums, the frequency of the multiphase clock signal has to be finely adjusted in some cases so as to correct variation in characteristics of the drums. In this case, it is inefficient to change the frequency of the reference clock signal for each pulse width modulation circuit, so the frequency of the multiphase clock signal to be output from the multiphase clock generating circuit 110 is changed. In view of the above, JP 2005-20083 A and US 2004/257124 each disclose a technology of changing the frequency of the clock signal to be generated, by using the multiphase clock signals.
FIG. 15 shows a block diagram of a clock generating circuit 202 disclosed in US 2004/257124, and description is given of the clock generating circuit 202. The clock generating circuit 202 inputs an output clock signal CLKO to a delay-locked loop (DLL) 208 and generates multiphase clock signals with 10 phases. Further, in response to a control signal output from a control circuit 203, any one of the multiphase clock signals is selected by a selector 209. Then, the selected clock signal (selected clock signal CLKS) is fed back.
With the above-mentioned configuration, the clock generating circuit 202 controls the frequency of the output clock signal CLKO to be set high in a case where the fed-back selected clock signal CLKS has a phase which is delayed from that of the output clock signal CLKO (or a reference clock signal CLKR). Specifically, the clock generating circuit 202 selects a clock signal to be fed back from among the multiphase clock signals, to thereby control the frequency of the output clock signal CLKO.
However, in the laser beam printer, not only the variation in characteristics of the drums but also variation in characteristics of a lens 102 is caused. The variation in characteristics of the lens 102 is caused during a process of producing the lens, and variation in strain characteristic is caused depending on positions in the lens. When the variation in the characteristic of the lens 102 is caused, there arises a problem in that, for example, a width and a position of a printing area p do not correspond to the pulse width of the PWM pulse, depending on a dot position.
In the prior art, the multiphase clock signals are generated using the delay-locked loop 208. Then, by using the generated multiphase clock signals, the frequency of the multiphase clock signal output from the multiphase clock generating circuit 110 can be changed.
However, as the results of study, the inventor(s) of the present invention has(have) found it difficult to control the frequency with enough precision to correct the variation in characteristic of the lens 102, at phase intervals between the multiphase clock signals generated using the delay-locked loop 208. Further, if the frequency of the multiphase clock signal is changed in response to the control signal sent from an external portion as in the control circuit 203, it is difficult to give a desired change characteristic (hereinafter, referred to as “frequency profile”) to the frequency of the multiphase clock signal according to the characteristics of the lens 102.